Modern manufacturing can include a process in which non-semiconductor elements such as lenses, optical fiber connectors and optical fibers are attached to semiconductor die. For example, some types of CCD or CMOS image sensor assemblies for use in still picture and motion cameras are manufactured by attaching a molded lens assembly to a semiconductor die in which an image sensor is fabricated. Currently, 500 image sensors can be fabricated on a 180-mm (6-inch) silicon wafer. Plastic lenses are typically gang molded, but each has to be individually mounted on an image sensor die after the wafer has been diced into individual image sensor die. Individual mounting is expensive and is subject to alignment errors.
Using a wafer-scale assembly process would reduce manufacturing cost and make the mounting process less error-prone. In the wafer-scale assembly process, an array of plastic lens assemblies would be molded with a pitch that exactly matches the pitch of the image sensors on the wafer. The array of lens assemblies would then be attached to the wafer to build hundreds of image sensor assemblies at a time. However, wafer-scale manufacture of such devices has been impractical hitherto because it is difficult to ensure that the pitch of an array of lens assembly molded using a multi-part mold accurately matches the pitch of the image sensors on the wafer.
The image sensors are defined photo-lithographically in the silicon wafer, which has a low coefficient of thermal expansion (CTE). The array of lens assemblies, on the other hand, is molded at a high temperature from a high CTE material. The pitch of the array of lens assemblies, measured between the centers of adjacent lenses, must match the pitch of the image sensor array, measured between corresponding points on adjacent image sensors, to an accuracy ±0.050 mm (50 μm). This alignment accuracy must be maintained across the width of the wafer. The molded array of lens assemblies is likely to have non-uniform shrinkage. Moreover, due to the higher CTE of the lens material, the pitch of the array of lens assemblies is more dependent on the temperature than the pitch of the image sensor array. Hence, the pitch of the array of lens assemblies is unlikely to match the pitch of the image sensor array with the required accuracy.
Thus, what is needed is a wafer-scale assembly method that provides high alignment accuracy between elements, such as image sensors, arrayed on a wafer and elements, such as lens assemblies, that are to be attached to the elements arrayed on the wafer. What is also needed is a wafer-scale assembly method that maintains the high alignment accuracy over the whole area of the wafer.